Depends...
Add a tee to the drain valve on the heater, add a tee to the hot water outlet side, fill the gap with hose and tubing to [under] your table [top]. Convection rulz, no pump needed. If it's close to your table, you'll get some heat. If it's enough though...
Toss some poly on a frame over the table. Set a small ceramic fan forced heater at one end of the 'tube', let 'er rip. Vary the temp by setting the heater closer/further from the opening. As in don't tape it to the poly tube/frame.
I've seen these heaters advertised as "personal" heaters, like under a desk to warm your tootsies. 500w tops, 250w on low (or thereabouts).
YMMV, don't forget to floss.
Rick