Abstract: A generally flat platform member is reinforced by a pair of support struts mounted to its bottom side, with a flexible fastener adjustably coupled between the two reinforcing struts by a tension clamp. The flexible fastener is adapted to extend generally parallel to the plane defined by the platform member, when mounted to a tree or other upstanding support, for securely mounting the platform member to the upstanding support. Advantageously, the support struts may be pivotally mounted to the platform member to accommodate upstanding supports of various sizes, and enable folding into a compact back-pack frame. The platform may also be formed with a rounded indentation along a portion of its back edge for abutting the support, and may include a reinforcing member thereat.