Bedding Down Inspection Chamber Cover Frame Drainage and wastage systems and plumbing help, advice and answers

[gotabelt]

What would be the best way to bed down a plastic type frame for a manhole cover frame. The frame is rectangular with rebates to allow the cover to sit in. I did consider a good bed of mastic sealant, but am not sure if this would work. There are no fixings or brackets on the frame.