This is the one that I use for my RG351V, but it looks like it's out of stock now:
https://www.amazon.com/gp/product/B08DLHKL2S/
The RG351V is approximately the same size and thickness as a 3.5 inch PC harddrive. Most carrying cases that advertise as being a 3.5 inch harddrive carrying case will work.
If you are interested, this is the case I got.
QIVYNSRY Portable Hard Drive Case HDD SDD 2.5 inch storage bag
https://www.amazon.com/dp/B08DLHKL2S/ref=cm_sw_r_cp_api_glt_fabc_VEVVHVHV890ZXJ7B6RT3
SanDisk 128GB Extreme microSDXC UHS-I Memory Card with Adapter - Up to 160MB/s
https://www.amazon.com/dp/B08DLHKL2S?psc=1&ref=ppx_yo2_dt_b_product_details
A case like this for hard drives or camera actually hold three gba games perfectly. I got one. the pocket fits snug.