I had Steve Earle make me some Unertl type mounts/rails about 4" long, with no cuts for Unertl mounts, just a dovetail rail.   

I use Tasco steel mounts intended for a grooved receiver, they clamp down on the rail perfectly.  I'll attach a picture.

You can use a Weaver or Picatinny rail, just look less attractive.  I did that once on a rolling block when I thought it was needed for load development.  Later I learned I could trust the iron sights, no scope needed.

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This is how my  44 1/2 Shuttleworth barreled 32-40 came to me, so I had no choice.
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Thanks to everyone for your suggestions. Tom

Yes if your rifle is set up for target blocks.  I simply use a set of one inch rings made for a grooved receiver.  As they are the rings I use are a loose fit so have to be shimmed. The shimming was simple. I found a set of brass letter stencils at the dump. They cut easily with scissors fold easily and are just the right thickness to get a good tight fit of the ring to the base . Just lay your cut out shim material on the base with about a quarter inch overhang on each side.  Press the over hang down on each side as far as you can with your finger then install the ring over the shim and done.  The one issue is the spacing of the target blocks.  They are usually spaced too far apart for modern short scopes. I like the old Weaver K model scopes for this purpose because they have a long tube. Other old model scopes with internal adjustments and long tubes are the Redfield 3200 and 3600 and the Lyman all American in the higher end magnification. I keep several Weaver K-10's on hand for this purpose.  As an example I recently picked up a Stevens rifle with factory target blocks and mounting an old Weaver K-10 in this fashion got me shooting right away.  If your not lucky enough to find a set of letter stencils at the dump then try using a pop can but with that you may have to double the thickness to get a good tight fit.  I can send you one of my stencils if you want.