Need a tensioning rod to push something tight (opposite of a turnbuckle)
Where we need to separate like you are mentioning we use what the millwright call jacking bolts.
These are heavy 1/2” or better. In some cases they use a threaded rod  (short distances up to about a foot).
If longer than a foot they use the same or larger thread stock and weld it onto a plate or tube steel the tube steel just a 2x2 can span 6’ - 8’.
The millwrights usually use national course threads but I have seen them use acme square on 2” thread stock for a heavier load (several tons) needed to be lifted but maintain adjustability.
I don’t know if there are other ways like a reversed turnbuckle but the millwrights use standard stock and the nuts and washers on the inside push the parts away from each other and they call them jacking bolts.