Re: 3/4" o.d. tubing
Go on E-Bay and look up seat posts. Some of the listings are from bicycle parts dealers that sell sleeves that convert your seat post to something bigger. The measurements are all in mm's though so you're gonna have to convert 3/4" to mm. If you don't want to go through the hastle, just get some thin wall tubing (maybe an exhaust coupler)and make your own sleeve.slit it down the side and cut off the extra after wrapping it around your 3/4" tube.
Just an idea....