You're right about the weight. Compared to your scope and mount, it's a beast.
You can use a t-adapter for the Pentax K bayonet to attach an eyepiece barrel-sized adapter to the camera. These are available here.
You'll want the #TADP and the #TTPK. They're shown here:
To use these with your scope, I'd advise using a diagonal to direct the optical axis parallel to your scope and resting the camera base on the OTA. When you adjust it for focus, you might need a block of foam between the camera and the OTA. If the focus goes the other direction, you'll likely need a barlow between the adapter and the focuser.
To provide better balance, slide your OTA more toward the rear of the mount rings. You may find this means that only the camera and focuser are forward of the rings, with most of the rest of the scope on the rear side.
You are only going to be able to photograph the Moon this way. Your mount is not acceptable for longer exposures needed for DSOs and the image of even Jupiter or Saturn given your focal length is going to be very, very small on film, even with a barlow.
But you can start out with the Afocal method, supporting your camera on a photo tripod and suspending it over the eyepiece. You'll find this easiest when the Moon is high in the sky. It takes a bit of practice to time it so the Moon is in the field of view for more than one shot. If you use this method, just be sure to keep the camera's focal plane as near as possible to perpendicular to the optical axis of the eyepiece tube adapter.
I think you'll find that using a webcam or something like the DSI, LPI, or NexImage cameras instead of a 35mm SLR will make this much, much easier and will certainly give you better results.