I also have this one and my RG350M fits perfectly in it. I got it on eBay for 5€ free shipping. Took a couple weeks, but I got it. Feels a little cheap, but it should protect it fine. If you want the luxury case, you can get this one: https://www.amazon.com/UGREEN-External-Passport-Expansion-10000mAh/dp/B072HJ4FTX/ which fits both RG350 and M perfectly.
The SD cards that come with it are prone to failure and the stock firmware sucks. If you have any micro SDs sitting around now, I'd recommend flashing 351elec to one. It's easy to do, instructions and downloads are at 351elec.de. Retro Game Corps on YouTube has easy to follow guides as well.
There are accessories. I bought this case for mine, there are also grips, vinyl skins and such if you search on Etsy. Depending how much you're willing to taking it apart there are replacement button mods available too.
It can seem overwhelming, but you can tinker as much or as little as you'd like.
These are the ones I've been looking at:
GLCON case https://www.amazon.com/dp/B082WYXD28/ref=cm_sw_r_em_apa_i_GAkpEbGPA2CP0
iMango Shockproof case https://www.amazon.com/dp/B01M66RT22/ref=cm_sw_r_em_apa_i_8CkpEbACB58XD
UGREEN Case https://www.amazon.com/dp/B072HJ4FTX/ref=cm_sw_r_em_apa_i_1FkpEbJ8DDNYW
Nice, I just ordered this one:
https://www.amazon.com/dp/B072HJ4FTX/ref=cm_sw_r_em_apa_i_IXYsEbDXFAH1V
This fits the 350 and 351mp, so something like this might fit the 353
https://www.amazon.com/gp/product/B072HJ4FTX
Just got this today for my new mighty. Another post here suggested it and everything fits perfectly!
I think I have this one. It's super snug, although it's more than double the thickness. I have a small USB charger inside in the other half and the charging cable, and nothing moves.