So the other day I noticed a small hole in the bottom of my pannier, its large enough to lose keys thru. Its right on the bottom panel. Now since im tight on cash, and the panniers overall are in good shape, I'm hoping to be able to just sew/patch the hole.

Anyone ever done this, have any suggestions to best go about this?
Was thinking of sewing what I could and maybe using an iron in patch, but I'm unsure my iron will fit

What's the bottom of the pannier made of? Just fabric? Some have thick, waterproof bottoms that would be hard to fix by sawing. Sawing a nice, large patch is probably a good idea. An iron patch would likely not be strong strong enough, so I'd still reinforce it with some stitches. A tent repair patch maybe? But, still not strong enough. I'd probably go with sawing, although you may glue to patch first with a fabric adhesive.

get a piece of rubber and some of that flimsy plastic sheet cut them to fit and glue them together with contact and then use some contact cement to glue to the bottom of the pannier. Maybe some of that Shoe Goo would work also. If these a canvas or denier fabric you should be able to sew them by hand, just make sure you are into good fabric to sew the patch in.

I use a Speedy Sticher for gear repair with heavy fabrics.

Yea its just water resistant fabric, not too thick. I've already sewn extra reflective material to the sides and back. Sewing in something with bit of thickness to the bottom panel would be nice.

Get some 1000D Cordura to cut patches from.
Cut an inside and outside patch at least 0.5" wider than the outer edge of the hole.
Use Gorilla Glue to bond the patches. It will not come apart.

I carry a tube of Gorilla Glue on long backcountry camping trips for gear repair. Done right, you can fix just about anything with that stuff.

I like denim for fixes like that, just a good piece from a worn pair of jeans or some such as that. Even a pair of jeans that has a hole in the knee or something will have a good spot somewhere. I like to sew it back together with light fishing line or dental floss, whichever I have at the time.

If you don't have one already go to the thrift store and buy a $1 backpack and have all the patch material you need and them some. Probably a choice of colors too.

Marine Goop
you want
Marine Goop

I've been using this stuff for years.
its great for a million little things

for your pannier repair, I'd probably use a pice of innertube, and just glue into position with Marine Goop
it will probably last longer than the rest of the pannier

I glued a Cars-r-coffins patch onto a messenger and I pulled it off with a pair of channel locks after 6 years

you can also thin any of the "Goop" products with Toluene
you just have to do it in a metal can, or glass jar
toluene will melt plastic
use in a well ventilated area.

when I apply Goop, typically I use a piece of wire, like coat hanger, old spoke, etc...

work with it, you'll the hang of it