Howie Glatter tuBlug™ 1.25"
The tuBlug™ is the answer for those who want to Barlow-collimate their primary from the back of the scope, but have a solid tube Newtonian, or can't be bothered to slide their shroud up.
It's a Blug (Barlowed laser plug), containing its own lens and shadow screen that fits within a side cut-out tube, the combination being known as a tuBlug. Like the collimator self-Barlow attachment or the Blug, it uses the Barlowed laser principle to project a silhouette shadow of the primary mirror collimation mark back onto its white screen, where the shadow is centered to collimate the primary. The tuBlug comes in 1¼" and 2" sizes, fits any focuser, and can be used with any well-centered laser collimator.