I've done it before, had them machined out of aluminium. The lower ones are fairly simple being just round bushes with a bolt hole, just make sure they are an interference fit in the subframe and you use big washers on the inner of the subframe, the top ones were more complex, a machined block, with chamfered edges on the lower long face ( aids fitting and is more aesthetically pleasing ), with a stepped edge on one upper long face to accommodate the rail flange, and a threaded bolt hole in the top,I bolted them through from the top with a long bolt rather than trying to fit a threaded rod into the ally block, though i suppose this would be easy to do in retrospect ( it was a bit of an *Bleep* getting the bolt holes to line up if i remember correctly, sometimes it would go, and sometimes it would not, like all viva bolts ).
This was fitted to a strengthened subframe (big washers welded to frame on all bolt holes, boxed out riggers, strengthened engine mounts,and strengthening plates) the extra strain on the subframe from being solid mounted could cause it to crack I guess, also had rose jointed adjustable tie rods and heavy duty rubber bushes on the wishbones, adjustable dampers wound all the way up and a heavy duty anti roll bar.
This kept to the old idea of 'hard on the front, soft on the back' for road holding and handling. ( i read that somewhere once) it seemed to work well.
But it gave one hell of a harsh ride, i wouldn't do it again unless it was for track use. It was a bit 'jarring' to say the least.
It was originally installed in this one, HB SL 'PUT' (I still own this one....albeit in many many bits )
Then it went in this one, HB deluxe 'TUE' ( this one spectaularly junked its gearbox on the way home from work at 7am, subsequently was torn apart by 'JS' and myself and the parts were scattered between his many HB's )
hope this helps