Gantry bearing mounting extrusion

[routercnc]

As Wal was eluding to it sounds like you are looking for a source of linear rail. For this application these sort of rails would work fine:
https://www.ebay.co.uk/itm/142713036010

You should be able to find the right length, if not you can cut them down with a thin inox disc on the grinder, but do it in stages to avoid getting it too hot and loosing the case hardness.

Search for SBR16 for the rail and SBR16UU for the carraige blocks. These are 16 mm size, if you want 12 mm size change the numbers to 12.

The dimensions of these family of rails are here, just measure from the gap between the lower surface (if that is where you want to mount them) to the gantry and get something which is close. If you are lucky the gap is a nice thickness that you can just order from aluminium warehouse (who will also cut to size). If not, as Wal says, get the next thickness up, get it cut to size and delivered, and some kind soul on here would probably skim in down on the lathe or mill for postage and coffee/beer fund money. I can't offer this service myself as every workshop minute is required to get my daughter's kart finished before she is too old to care (!)



Take some measurements, mock up some cardboard, whatever you need, and see if this works.

You then need to link it down to the belt drive. A bit of angle aluminium/iron off the edge of the shim plate on top of the carriage should do it.