I would like to have an acrylic tank made specifically for an octo.

Does anyone have any good sure fire designs to keep an octo secure inside?

I am thinking of having the display with a sump and skimmer.

So I would have a drain, and then a return pump in the sump with a skimmer down there.

There are plenty of past treads on this subject that should be some help to you.

I just ordered an acrylic tank from glasscages.com specifically for an octo setup. When the owner called to confirm my order I asked him if the acrylic tops fit tight or was there a gap in the back like all glass. He told me it was a tight fitting lid and it would need to be trimmed for filters, etc. I'll post a pic when mine comes in. You may need to add some weight to the top however.

how large was is the tank, and what did it cost you if you dont mind me asking?

36x18x17. $235 after several customizations, bulkheads, tops and freight.