Large building, HDM and measurement....

I'm trying to build a cylindrical tower. The standard hollowed cylinder prim is of too small an internal radius to be useful to me. I need a bigger radius.

I've tried HDM/huge prims from several sources, but I can't make them habitable. Path Cut creates a door, but I can't walk through it; the huge cylinder acts as if it's not been Cut or hollowed. I've tried this with numerous sizes and shapes of HDM prims, and it seems to be consistent across all of them. I can walk across the top of a hollow HDM 20x20x20 cylinder, for example, even though there's nothing visible beneath my feet. Is this always the case, or can I make an oversized hollow cylinder that actually behaves as if it's hollow?

I tried making the tower from segments of cylinders patched together with rectangular prims, but the prim count quickly goes through the roof.

Failing the HDM route, how might I make a cylindrical structure that doesn't take up an obscene number of prims? Thanks.

EDIT: I know I can make set a megaprim to "phantom," but I've been in at least one cylindrical building with a solid wall made from a single cylinder bigger than 10x10.

try editing the Mega in place. I've noticed issues with the collision plane ignoring cuts and hollows when the the object is moved or moving. if that doesn't work try setting then unsetting phantom, or force a ful prim update by giving it hover text (you can take it away right after)

Thanks for the suggestions. I tried them all and still have the same problem. Any megaprim I rez can be cut or hollowed, but the insides still behave as if solid.

I went back and looked at a structure I remembered seeing from some time back, and it is indeed made of a 25'x25'x10 cylinder, cut about 10-15% and hollowed maybe 80-85%. The cylinder is a single prim (I did an Edit to check), not phantom (can't walk through it) and not surrounded by invisible prim plating or any other trick for making phantoms seem solid. I'm baffled. I can't get in touch with the creator to ask how he did it.

to be honest I've never had a problem with them, including cuts and hollows so I'm not sure... maybe get a new set from onrez or slx (they're listed free on both last I checked) could be a corrupt copy

The bad news: In my experience, megaprims have a high tendency to ignore their shape and be physically defined mostly by their bounding boxes...

The good news: ...until you move those prims into a Havok 4 simulator.

Apparently our big incoming physics upgrade means a whole lot less trouble handling things like gigantic, oversized shapes.