If you're looking for a rail just for the red dot, rails sold on amazon for around $15 will work fine.

However, if you want more space for a magnifier also, the slightly more expensive longer mounts will require you to use a dremel to make a bit of space on one side so that it sits flush on your mp5. I recommend you compare the bottom images of the rails from hkparts.net and ones from amazon.

amazon - standard rail for red dot

https://www.amazon.com/dp/B07P9MN9PY?psc=1&ref=ppx_yo2ov_dt_b_product_details

amazon - longer rail

https://www.amazon.com/dp/B086DL4YVY?psc=1&ref=ppx_yo2ov_dt_b_product_details