As an important reason why I play on RecalBoxed Pi with the great RecalBox RGB Dual add-on, is because it can handle RetroAchievements and CRT display (and as such, is superior to a MisterFPGA solution - hey, just pushing a joke here ðŸ¤), I've been quite bothered about how unreadable the RetroArch notifications are with this setup (all hyper stretched vertically, due to the specific resolution ratio used by the add-on, in coordination with the way RetroArch handles notifications display).
So I played with the various RetroArch settings on messages/notifications and found a solution.
It's an inferior one compared to the nice RetroAchievements widget, but at least it's a readable one.
It mainly involves switching back to classic text only notifications (which are also static, not scrolling).
Here is the result I got with my configuration:
To get this, I created a .retroarch.cfg RetroArch configuration file in /recalbox/share/ directory (that's \\RECALBOX\share\ when accessing using samba share),
with the following content :
menu_enable_widgets = "false" video_message_color = "ffff20" video_msg_bgcolor_enable = "true" video_msg_bgcolor_red = "10" video_msg_bgcolor_green = "10" video_msg_bgcolor_blue = "70" video_msg_bgcolor_opacity = "0.5" video_font_path = "/recalbox/share/fonts/VIENNAHY.TTF" video_font_size = "25" video_message_pos_x = "0.04" video_message_pos_y = "0.06"
While editing this file, If like me you also got problems with RetroAchievements "hardcore" mode not activating on some cores (like PlayStation), despite having it configured as such on RecalBox configuration menu, you can also add this entry (anywhere in the same configuration file) that will force "hardcore" mode (the RecalBox mode setting from the configuration menu is then bypassed, please note that) :
cheevos_hardcore_mode_enable = "true"
Notice that a specific font is being used on the video_font_path parameter, not provided with RetroArch or RecalBox, but key to get readable notifications.
It's this font : https://www.1001fonts.com/gf-vienna-heavy-font.html
So you need to download the font here, getting a gf-vienna-heavy.zip file.
Then unzip that file and copy the VIENNAHY.TTF font file in a new directory on your Pi: /recalbox/share/fonts/ (so create a new fonts directory in /recalbox/share/ and copy that file in it).
As you can see on its webpage preview, that this font is quite stretched horizontally, so... it compensates the vertical stretch induced by the RGB Dual resolution. 
And that's about it.
You can change the message colors if you don't like those I used. Just experiment with the various color parameters in the configuration file, background opacity, etc.
Also you can see that the message background isn't exactly behind the message text, but nothing can be done on that, it's like that with any font, even the default RetroArch one.
@dma-sc thanks you so much, I just try it and it work perfectly.
I will see for add this configuration in the next release of recalbox
@beudbeud Ah cool, thanks for testing this. 
As you're talking about including it in the distrib, I see a potential problem with the used font's license.
So I went an extra mile and modified an existing font included in RecalBox (the "unbuntu_condensed" one).
Here is the result (some software provides automatic functions for this and is intuitive enough, I used "fontforge"): https://github.com/mstempell/ubuntu-condensed-4x-wide/raw/main/ubuntu_condensed-4x_wide.ttf
And here is how it looks like with this modified font:
For people willing to use it directly, after following the instructions from my previous post, just put this new font file in the /recalbox/share/fonts/ directory and change the video_font_path parameter to:
video_font_path = "/recalbox/share/fonts/ubuntu_condensed-4x_wide.ttf"
The vertical position of the message/notification can also be lower with this last font actually, like setting it to :
video_message_pos_y = "0.05"
(or even to "0.04" if you don't mind have descender part of letters g-j-p-q-y, half cropped on some RetroArch cores resolution ratios)
@dma-sc thanks for all, I pushed code in next version
Glad to be of help!
@dma-sc thanks so much for your solution! 
I have a similar issue on my recalbox pc with radeon graphics. Have tried changing the fonts, first to vienna and then to ubuntu
This is my issue:
The vertical position of the message/notification can also be lower with this last font actually, like setting it to :
video_message_pos_y = "0.05"
(or even to "0.04" if you don't mind have descender part of letters g-j-p-q-y, half cropped on some RetroArch cores resolution ratios)
