Inspection chamber sizes

[chapcot]

Laying a new path and need to raise the height of a sewage inspection chamber but can't seem to find the correct size of riser. The existing chamber (probably installed around 1989) has an outside diameter of approx 490mm and an inside dia of approx 482mm, but the only risers I can find are quoted as 450mm dia. Has anyone any idea where I might source the correct size or is it possible to adapt the existing chamber to a 450mm riser??
Many thanks for any assistance.

 

[noseall]

It is possible to reduce the size of the aperture by using pcc lintels provided you have the tolerance for the thickness of lintel, lid etc.

 

[BBSBuilding]

other than that...the drainage center have a large range but they are quite expensive.