I have a 1960's Weaver D4 mounted for now, on an aluminum rail I substituted for the plastic one (which had cracked around the middle bolt due to factory over-tightening) after drilling out and tapping the holes carefully for a pair of 8-32 bolts. The scope is a lot too long when folding the rifle, but I have a pair of trim little Weaver steel rings on it with thumbscrew QD so it's easy enough to pull the scope off for transport, and it seems to return to zero between mountings.
I'll use the same rings when the 'proper' scope gets here from an eBay seller. I know... buying a thing from China... but hey, I really tried to find a compact scope for it locally and nobody seems to sell anything appropriate for a Little Badger. So I ordered one of these things for about $80CAD with shipping:
https://www.ebay.ca/itm/Ohhunt-4-5x...al-Sight-P4-Glass-Etched-Reticle/324018571782
At about 6.5" long (165mm) it's more appropriately sized for the tiny Badger, where the old Weaver scope is almost a foot long and kind of silly looking, while having too short eye relief at about 40mm. This little 4.33x scope pushes the eye back to about 66mm. Still not enough to make it ultra-compact when folded, it'll be hanging off the back, but maybe enough that I can just leave it in place comfortably. I've made a couple of other changes including dismantling and polishing things, greasing properly, shortening the LOP a little by carving out the body and shortening the two steel rods 1/2", odds and ends to make it 'mine.'
The biggest discovery I made was that the star washer they used in the main joint was basically
eating the magnesium alloy of the body, loosening it with use, leaving tiny slivers of the metal in the hinge under the washer. Got rid of that. Put in a flat steel washer and used blue Loc-tite to secure the main bolt. Now it doesn't get looser as it's just pivoting on a well greased joint, not grinding itself to death. I'd recommend this change for all owners.
As for properly answering the question, sorry, I can't see any after-market open sights which might work as a simple add-on. Most things will require some drilling and thread tapping to make them work. My own rear sight arrived twisted at a funny angle, and when I dismantled the thing to find out why, it was obvious the two tiny plastic pins which were moulded into it just sheared off at some point during transit. The remaining bolt in the middle just let it spin. Not much use as a sight that way! Some things about this thing are incredibly awful, but overall it's a wonderful platform for expression of one's amateur gunsmithing. Polishing out the chamber with some 400grit paper wrapped around a dowel and chucked in a drill let spent cases be pulled out without pliers too, like it was as supplied. After half a minute of running the sandpaper covered dowel in and out with the drill it was then easy to withdraw brass between thumb and finger.