Apologies BB. Thank you for your efforts.
.bundle zip file reattached with full contents below:
Many thanks, Robbo.
Got it working using this for PackageContents.xml file:
<?xml version="1.0" encoding="utf-8"?>
<ApplicationPackage
SchemaVersion="1.0" AutodeskProduct="AutoCAD" Name="IRR-SteelSectionsToolPalette" Description="Steel Sections ToolPalette" AppVersion="1.0.0" ProductType="Application" AppNameSpace="" Author="Robbo" ProductCode="{3DC11CE4-CF92-45A0-B123-D566E691AB0A}" UpgradeCode="{42E3AEE8-6F93-41F3-AEF0-B9F06E65D22C}">
<CompanyDetails Name="" Email="ROBBO@TheSwamp.org" />
<Components>
<RuntimeRequirements OS="Win64|Win32" Platform="AutoCAD*" SeriesMax="R19.1" SeriesMin="R19.1" SupportPath="./Contents/Support" ToolPalettePath="./Contents" />
</Components>
</ApplicationPackage>
It appears that the ToolPalettePath XmlAttribute also accepts semicolon separated strings (for multiple paths); perhaps adding SupportPath XmlAttribute is not needed. *not sure*
Just to clarify, I initially tested this (adding the tool palette path, and the block's location within .bundle to the ToolPalettePath XmlAttribute), and it fails.
More specifically, the tool palette is loaded just fine, but the blocks fails to insert from palette, resulting in the following error:
Source drawing "./Contents/Suppport/Steel Sections.dwg" for block "Steel Channel" not found.
I then, removed the block's location from ToolPalettePath XmlAttribute (semicolon separated list), and instead added the block's location to SupportPath XmlAttribute as shown here (above), and all is working well actually.
Cheers