I've been loading and shooting 43 Mauser for several years.
When I got my 71/84 I was in the same boat as you. Was having trouble finding brass and when I did they were $5 plus per piece.
So being the cheap bastard that I am I started measuring rounds that were close.
What I came up with was 45-90 was very close in length (slightly too long), the base was a tad small and the rims were about 30 thou too large in diameter.
When I started this project I didn't have a lathe so I did everything by hand.
After a quick annealing I ran the shells through a FL sizing die to neck down the brass and put a bit of a shoulder on them.
After that I found a bushing that fit the neck so that I could put them in the chuck of my drill press without collapsing the necks. Then turned on my drill press and ran a file on the ouside diameter of the rims until they fit the diameter bolt face.
After that I put a bevel on the rims so that them would fit the concave shape of the bolt face. The other option that I considered was to chuck up the bolt into a lathe and relieve the face til it was flat right across meaning that a factory 45-90 piece of brass could chamber without putting a bevel on the case head.
Once I want to shoot the new brass I slip an o-ring over the case and push it down to the rim which takes up any headspace slop and hold the rim firmly against the extractor.
After that it's just a matter of fireforming them which blows out the Web of the case quite a bit. They expand a bit but I have never lost a single piece when forming them.
Upon second firing I don't use the o-ring anymore and usually I only slightly resize the neck as I find a FL resizing isn't nessesary.
For bullets I use Lee .459" diameter 405 grain hollow base bullets sized down to .447" with a custom sizer I made.
I used IMR 4198 powder at 28 grains with a dacron filler to hold the powder against the primer.
Accuracy with this load is about 2-3 moa and is quite pleasant to shoot.
Those are 45-90 brass that I made into 43 Mauser. They shoot and chamber just find and the gun can't tell the difference.
OP of you go this route and have any issues send me a PM I'd be more then happy to help you out.
The process is pretty easy once you've made a couple and the best part is with my method brass is about $1-$2 piece
And the brass seems to last forever. I've loaded my brass at least 10x with a single issue.