Is there a particular reason for using forced or hothouse rhubarb rather than picking out of the garden?
Nigella's Toasted Marshmallow And Rhubarb Cake (from COOK, EAT, REPEAT) has a filling made from roasted forced rhubarb (known as hothouse rhubarb in the US). Forced rhubarb is grown in dark conditions and the slim stalks are more tender than regular rhubarb, as well as being more delicate in texture and more vibrant in taste and colour. Regular summer rhubarb tends to be more fibrous and has a tendency to collapse into a khaki mush when cooked.
If forced rhubarb is not available then you can use 400g (approx. 2 2/3 cups) fresh raspberries as an alternative. Crush a few with a fork to a rough puree and spread this over the bottom cake layer. Dot over some whole raspberrries before sandwiching with the top cake layer. Serve the remaining raspberries alongside slices of the finished cake.